In-plane switching (IPS) LCD uses thin film transistor (TFT) technology to improve image quality. The IPS LCD delivers bright pictures with very good color consistency at a wide viewing angle. IPS LCDs are used in television sets, computer monitors, mobile phones, handheld systems, personal digital assistants, navigation systems, projectors, and many other devices.
An IPS LCD includes an array of pixels for displaying images. The pixels are addressed in rows and columns, reducing the connection count from millions for each individual pixel to thousands. The column and row wires attach to transistor switches, one transistor for each pixel. The one-way current passing characteristic of the transistor prevents the charge applied to the pixel from draining between refreshes to the display image.
In an IPS LCD, the liquid crystal extends horizontally across the panel and essentially provides a wide viewing angle, fast response speed, and a simple pixel structure. The IPS LCD employs pairs of electrodes at the sides of each cell, applying an electric field horizontally through the material. This approach keeps the liquid crystals parallel to the front of the panel, thereby increasing the viewing angle.
FIG. 1A illustrates a perspective view of an electronic device, such as an IPAD. The electronic device includes a touch screen display 100 enclosed by a housing 138. The touch screen display 100 includes a touch panel 102 on a front and an LCD display behind the touch panel 102, although alternative embodiments may employ an OLED layer instead of an LCD. A cross-section is taken along line 2-2 in FIG. 1A. FIG. 1B illustrates a simplified cross-section diagram for the touch screen display of FIG. 1A. Touch screen display 100 includes a touch panel 102 above an IPS LCD 104. The touch screen display 100 may have an air gap 106 between the touch panel 102 and the IPS LCD 104. Alternatively, in a full lamination design, an optically clear adhesive (OCA) may connect the touch panel and the LCD such that there is no air gap between the touch panel and display.
FIG. 1C illustrates a cross-section of an embodiment of an IPS LCD of FIG. 1B. The IPS LCD 104 includes a front polarizer 118, a rear polarizer 108, and liquid crystal layer 112 between the front and rear polarizers. The IPS LCD 104 also includes TFT layer arranged between the liquid crystal layer 112 and the rear polarizer 108. The IPS LCD 104 further includes color filter (CF) layer or glass 114 arranged between the front polarizer 118 and the liquid crystal layer 112. The IPS LCD 104 further includes a backlight 130 configured to provide white light to the rear polarizer 108.
The IPS LCD usually does not have common electrodes on the color filter (CF) glass, and so is vulnerable to electrostatic discharge (ESD). A conducting coating, for example, indium-tin oxide (ITO) coating, is often put on the top surface of the CF glass to help reduce vulnerability to ESD.
The IPS LCD 104 may also include an ITO coating 116 on a top surface of the CF glass 114, such that the front polarizer 118 is disposed over the ITO coating 116. The ITO coating 116 also provides shielding to the touch panel 102 from the TFT layer 110. The front polarizer 118 may include an adhesive layer 136, one or more optical films and/or compensation films 134, a polyvinyl alcohol (PVA) with an iodine doping layer 126, and a plastic film 128, such as triacetycellulose (TAC), cyclo-olefin polymer (COP), poly(ethylene terephthalate) (PET) or Poly(methyl methacrylate) (PMMA) film. The PVA absorbs light forming particular polarizers.
Generally, noise may be coupled from the IPS LCD 104 to the touch panel 102. When the stackup of the touch panel and the IPS LCD becomes thinner, the noise in the touch panel may increase. In order to provide better shielding, the ITO coating may need to be thicker. However, optical transmittance may be reduced as a result of increasing thickness of the ITO coating. Acquiring both lower noise (or higher shielding) and higher light transmittance (or lower reflection) becomes challenges for thinner touch screen displays.
There may be a trade-off between aspects of product design and touch performance. Basically, it may be desirable not only to reduce product thickness, which may result in the touch panel and the LCD being closer to each other, but also to reduce light reflection from the front of the display. However, touch screen performance and operation may be affected by electrical noise.
There remains a need for developing techniques to resolve the above issues to meet the customer needs of new touch screen display products.